Method and apparatus for arc detection and protection for electronic ballasts

ABSTRACT

An electric arc detection apparatus and method is based on AC current rectification phenomena in air plasma that causes low frequency amplitude modulation of high frequency currents and voltages in the ballast when disconnecting the lamp (Lamp) from the electronic ballast with power applied. A protection circuit (R 25 , C 27 , D 44 , D 45 , C 28 , C 27 , M 4 ) shuts off the inverter (M 1  and M 2 ) of the ballast so the duration of the arc is diminished so that the arc becomes almost non-visible. The protection circuit senses the input of the ballast inverter resonant tank (C 3  and L 1 ), which is free of transients caused by resonance, detecting arc rectification frequency which is about 25-30 times less than the inverter carrier frequency, and turns on a switching device (M 4 ) for stopping oscillations in the inverter (M 1  and M 2 ). When the lamp (Lamp) is reconnected to the ballast, it resets the protection circuit and the ballast inverter restarts automatically.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to electronic ballasts for powering a highfrequency electrodeless fluorescent lamp. An electric arc appears in thelamp connector when disconnecting the lamp under power. It isdestructive to the ballast and dangerous to the personnel replacinglamps. Also, arcing may be caused by poor connections in the fluorescentlamp wiring or disconnecting of crimped wire from the connector and maycreate a fire hazard.

[0003] 2. Description of the Related Art

[0004] Since the fluorescent lamp is powered from a current source withhigh operating frequency (250 kHz or more), such as a self oscillatingDC to AC inverter, a stable arc path is established between connectorpins or between the connector pin and a lamp wire, even if two separatedportions are placed by a distance of up to 1-2 inches from each other.At high frequency, recombination time of particles (electrons and ions)in arc plasma becomes comparable with AC frequency. When crossing zerocurrent, it is not enough time for particles to be recombined in gasmolecules and to stop the current flow and cancel the arc. Therefore, itshould be done artificially by stopping oscillation in the ballastinverter with a shut down circuit susceptible to the arc.

[0005] There are a few known drawbacks, however. When the arc appears inthe connector, there is no actual change in ballast high frequencyvoltages and currents that could be used for arc detection, as thevoltage drop across the arc is negligible in relation to lamp ratedvoltage. Additionally, there is a large increase in ballast voltages andcurrents during normal lamp starting and they are also effected by lowfrequency 100/120 Hz steady-state modulation caused by the AC linerectifier. To avoid false responses, the arc detection circuit shouldnot be susceptible to all of these disturbances, which occur duringnormal lamp operation.

[0006] The prior art teaches arc cancellation in the lamp connector bymechanically interlocking the ballast inverter when unplugging the lamp.In some ICETRON/ENDURA electrodeless lamps, additional pins are used inthe connector to disconnected some components of the ballast inverterwithout which oscillations in the inverter cannot exist. However, therequired three-wire connector is thick, expensive, and not applicablefor a lamp that is placed a distance from the ballast.

[0007] Other references disclose different sensing means for arcdetection, but they are only associated with low frequency AC devices,like electrical welding equipment, and not applicable for an arc in highfrequency ballasts. Furthermore, the purpose of such devices is arcstability. In contrast, the purpose of the present invention in aballast is arc cancellation.

[0008] Therefore, a protection method and circuit with fast arccancellation capability is still needed. Another feature of theprotection circuit should be a reset capability for restarting areconnected lamp. The protection circuit is also required in the eventthat the ballast is mistakenly turned on without a lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1a illustrates arc current and ballast output voltage plotsin transition from regular operation to arc condition caused byunplugging an electrodeless lamp.

[0010]FIG. 1b illustrates the same parameters as those in FIG. 1a at apoint when current rectification in the arc is starting.

[0011]FIG. 2 shows a ballast circuit diagram with a block diagram of anarc detection and cancellation circuit of the present invention.

[0012]FIG. 3 shows a circuit diagram of ballast with a self oscillatinginverter and an arc cancellation circuit.

[0013]FIG. 4 shows an arc detection and cancellation circuit with anotch filter.

[0014]FIG. 5 illustrates arc current and output voltage plots of theballast with arc cancellation, taken when unplugging an electrodelesslamp.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] In FIG. 1a, the upper plot is ballast output voltage V_(out) andthe bottom plot is ballast output current I_(out) powering a lamp via aconnector when the ballast is not provided with arc protection. The leftside of the plots represents normal ballast operation with a lampplugged in to the connector, just starting its movement away from theconnector. It is at the very beginning of arcing, when the gap betweenconnector pins is very small, that low voltage can break the gap. As theelements between which the arc has formed move further, the currentwaveform changes. Small steps are evident in arc AC current I_(out) atzero crossings. This represents a beginning of the recombination processin plasma. But plasma in the gap still continues breaking in bothdirections by AC output voltage.

[0016] As the gap further increases, the recombination process advances,so air-plasma mixture in the gap stops breaking in one direction. Thisis shown by intervals in which high frequency current pulses followrandomly in one direction only. Depending on the concentration ofparticles in different spots of plasma, it call be broken in onedirection and unbroken in the opposite direction. This means that aconnector pin may operate as a cathode and the opposite pin as an anode,or vise versa. Accordingly, when the arc conducts, the ballast resonantcapacitor provides extra current to the lamp in one direction and whenit does not conduct, the capacitor absorbs extra current in the oppositedirection. Therefore, a low frequency component appears in the outputballast voltage V_(out).

[0017] Further, low frequency oscillations create a situation in whichhigher absolute peak voltages are being applied in one direction andlower absolute peak voltages are being applied in the oppositedirection. Therefore, the air/plasma mixture has a tendency of breakingin one direction (rectification effect). The low frequency oscillationsbecome relatively stable, as illustrated in FIG. 5. This system can beclassified as an oscillator with negative impedance wherein theair/plasma mixture represents this negative impedance.

[0018] In the inverter of FIG. 2, low frequency amplitude modulationcaused by arc rectification affects almost all voltages and currents inthe ballast. However, these voltages and currents are inconvenient forarc detection since they are also affected by resonance during normallamp starting. According to the invention, inverter resonance tank inputvoltage V_(ac) (see FIG. 2) is utilized for detecting the arc in theconnector, since this voltage is directly affected by arc rectificationand is not affected by resonance.

[0019] The arc detection method is based on detection of the ACrectification phenomena that characterizes an electrical arc inair-plasma when it is powered from an AC current source. The abovemethod comprises steps of generating an arc through an air gap as aresult of disconnecting the lamp from an operating ballast,alternatively rectifying positive and negative ballast voltage pulses bythe arc, generating low frequency amplitude modulation of the ballastoutput voltage, sensing input voltage of ballast inverter resonant tank,determining a low frequency signal of the voltage corresponding to therectification frequency, filtering out such signal from all othersignals applied to the resonant tank, rectifying the signal, and holdingenergy of the rectified signal for a few of its periods. This resultingsignal is utilized to shut down the ballast inverter and cancel the arc.

[0020] The circuit illustrated in FIG. 2 includes elements of a knownballast circuit including a DC/AC inverter connected between a DC powersource and a lamp connector. The DC power source may be a rectified ACsource, a battery, or any other source of DC power.

[0021] The DC/AC inverter includes a capacitor C25 connected betweencommon and DC voltage +Vbus. Also connected between +Vbus and common areswitching transistors M1 and M2. The gates of M1 and M2 are separatelyconnected through resistors R16 and R15, respectively, to outputs of aninverter control circuit. The point between M1 and M2 is connected to afirst terminal of DC capacitor C1. A series resonant tank circuit isconnected between a second terminal of C1 and common.

[0022] The series resonant tank circuit includes inductor L1 andcapacitor C3. Lamp connector pins P6 and P8 are connected to respectiveterminals of the series resonant tank capacitor C3. A feedback circuitis connected between a point between L1 and C3 in the resonant tankcircuit and an input of the inverter control circuit.

[0023] These features of a DC/AC inverter are known in the art.

[0024] The arc detection and cancellation circuit of the presentinvention (as illustrated in FIG. 2) includes a low pass signal filtercircuit sensing the voltage V_(ac) at the input of the inverter resonanttank designed to select low frequency voltage signal components thatcorresponds to arc rectification frequency, a rectifier connected to theoutput of the filter circuit for rectifying this voltage signal, anenergy storage circuit for holding energy of these signals, a thresholddevice for noise immunity, and a latching switching device for shut downof the inverter and PFC through a disable terminal via diodes D47 andD49.

[0025] In the embodiment illustrated in FIG. 3, a self oscillatinginverter is formed by switching transistors M1 and M2 driven by afeedback transformer T9, DC capacitor C1, and the series resonant tankwith inductor L1 and capacitor C3. An electrodeless lamp is connected inparallel to the resonant capacitor C3 through connector pins P6 and P8.The inverter start circuit comprises a discharge capacitor C13, a diacX28 and a resistor R6 connected to positive bus rail +Vbus. DC busvoltage is formed by a boost type AC/DC converter. It can be a powerfactor corrector (PFC), driven by a PFC controller (not shown in FIG.3). An arc detection and shut down circuit comprises a low pass signalfilter (R25, C27), a rectifier of the low frequency signal caused by therectification process in the arc (diodes D44, D45), a storage capacitorC28, a discharge resistor R27 and, a switching transistor M4.

[0026] An advanced arc detection and shut down circuit illustrated inFIG. 4 comprises a low pass notch filter that is formed as a seriescombination of a RC low pass signal filter (R31, C29) and a lowfrequency block signal filter (R33, C31). This circuit has an inputterminal A and an output disable terminal B corresponding to theterminals with the same designations of FIG. 3. The notch filter istuned up to pass the low frequency signal generated by the electricalarc.

[0027] During normal operation of the ballast in FIG. 3, high carrierfrequency rectangular voltage V_(ac) is applied to input A of the arcdetection circuit. This voltage is filtered out by low pass filterR25/C27. As a result, voltage across capacitor C27 is well below a diodedrop voltage and has no effect on the input of the transistor M4. Whenarcing occurs and a low frequency rectification begins in the arc, a lowfrequency amplitude modulation is superposed on the high frequencyvoltage V_(ac). RC filter R25/C27 has a low loss regarding a modulationfrequency that is, at least, an order less than that of the carrierfrequency, so that frequencies associated with normal lamp operation arefiltered out. As an example, in the case of an ICETRON/ENDURAelectrodeless lamp having carrier frequency of about 250 kHz, modulationfrequency in the arcing connector is in the range of about 8-10 kHz. Itcreates a low frequency signal at the “A” input having a peak to peakvoltage of a few tens of volts that is attenuated by the filter.

[0028] At least a few volts of the low frequency signal is appliedacross the diode D44. In the circuit of FIG. 3, for reasons ofsimplicity only a positive wave of the signal charges the capacitor C28via the diode D44. The negative wave is shorted by the diode D44. Whenvoltage across the gate of the transistor M4 reaches the turn-onthreshold of the transistor, the transistor M4 starts being turned “on”with low frequency. It creates more disturbances at the input “A” of thearc detection circuit as well as higher voltage across the capacitorC28, and ultimately stops switching of the transistors M1 and M2. Thecapacitor C28 stores voltage that keeps the transistor M4 in the “on”condition during the recombination process of electrical particles inplasma. When impedance in the air gap changes from low to high, alatching signal from DC bus via the resistor R24 applies to the input“A” of the arc detection circuit.

[0029] The diode D44 can be selected as Zener diode that protects thegate of the transistor M4 from over voltage. Since Zener diodes havehigh parasitic capacitance, the capacitor C29 can be omitted. Theshutdown transistor M4 shorts out the capacitor C13 through the disableterminal B via a diode D46 and limiting resistor R30, preventing theballast from restarting the inverter after the shutdown. It also shutsoff the PFC controller (not shown in FIG. 3) via a diode D49 andprovides a reset capability. When the lamp is reconnected, it couplesinput “A” of the arc detection circuit to the “common”, the capacitorC28 discharges via the resistor R27, and the shut down transistor M4turns off, releasing the PFC controller and the capacitor C13 thatcharges and turns on the diac X28.

[0030] The operation manner of the arc detection circuit of FIG. 4 issimilar to that of FIG. 3. By comparison, the input filter in FIG. 4provides more noise immunity against transients generated during theinverter start up and against 100/120 Hz ripple coming from the AC line.Beyond that, R31 and C29 correspond to the filter formed by R25 and C27;D51 and D50 correspond to rectifying diodes D44 and D45; C30 correspondsto storage capacitor C28; R32 corresponds to discharge resistor R27; andM5 corresponds to shutdown transistor M4.

[0031] The further useful feature of the arc detection and shut downcircuit in FIG. 3 is its ability to interlock the ballast start circuitwhen the ballast is powered on without a lamp connected. This circuitshorts out starting capacitor C13 before it is charged to the thresholdvoltage of diac X28.

[0032] As an example of a low cost solution for an ICETRON/ENDURA arcdetection and shut down circuit such as that illustrated in FIG. 3, thefollowing components can be used: R24-1 MΩ; R25 and R27-470 kΩ; C27-1nF; C28-470 pF; D44-1N5248B; D45, D46, and D49-1N4148; D47-IN4005GP;M4-IRFD014. With the above components, it takes about 5 msec to cancelan arc caused by disconnecting the lamp from the ballast, as illustratedin FIG. 5. This makes the arc non-visible and not dangerous.

[0033] The embodiments described above are intended to be illustrativeand not limiting. It is recognized that various equivalents,alternatives, and modifications are possible within the scope of theappended claims.

What is claimed is:
 1. An electronic ballast circuit for a lamp with arcdetection comprising: a DC/AC inverter; a ballast output lamp connector;a low pass signal filter having an input electrically connected to theDC/AC inverter; a rectifier electrically connected to the low passsignal filter; an energy storage element electrically connected toreceive a rectified output of the low pass signal filter; a switchingdevice electrically connected to the energy storage element and to adisable terminal of the electronic ballast circuit; wherein theelectronic ballast circuit is configured so that during normal operationof the lamp, the energy storage element is charged to a voltage lessthan a voltage required to close the switching device, and wherein undera condition of electrical arcing from the lamp connector of theelectronic ballast, the energy storage element is charged to a voltageat least as high as the voltage required to close the switching deviceand activate the disable terminal.
 2. The electronic ballast circuit ofclaim 1, wherein the disable terminal is connected through a switchingtransistor disabling diode to a gate of at least one of the switchingtransistors.
 3. The electronic ballast circuit of claim 1, wherein theDC/AC inverter comprises an inverter control circuit, and wherein thedisable terminal is connected through an inverter control circuitdisabling diode to the inverter control circuit.
 4. The electronicballast circuit of claim 1, wherein the electronic ballast circuitfurther comprises a power factor correction element, the disableterminal being connected through a PFC disabling diode to the powerfactor correction element.
 5. The electronic ballast circuit of claim 1,wherein the low pass signal filter comprises a filtering resistor and afiltering capacitor.
 6. The electronic ballast circuit of claim 1,wherein the rectifier comprises at least one diode.
 7. The electronicballast circuit of claim 6, wherein the rectifier comprises a firstrectifying diode connected in parallel with the filtering capacitor. 8.The electronic ballast circuit of claim 7, wherein the rectifiercomprises a second rectifying diode electrically connected to the energystorage element.
 9. The electronic ballast circuit of claim 8, whereinthe first rectifying diode is a Zener diode.
 10. The electronic ballastcircuit of claim 1, wherein the energy storage element comprises astorage capacitor.
 11. The electronic ballast circuit of claim 1,wherein the switching device comprises a transistor connected betweenthe output node and ground.
 12. The electronic ballast circuit of claim11, wherein the energy storage element is connected to a controlterminal of the transistor.
 13. The electronic ballast circuit of claim12, wherein the switching device is a FET and the energy storage elementis a capacitor connected to a gate of the FET.
 14. The electronicballast circuit of claim 1, wherein the low pass signal filter is a lowpass notch signal filter comprising a low frequency block signal filter.15. The electronic ballast circuit of claim 14, wherein the lowfrequency block signal filter comprises a low frequency block resistorand a low frequency block capacitor connected in parallel with oneanother.
 16. An arc detection circuit for an electronic lamp ballastcomprising: an input node; a low pass signal filter electricallyconnected to the input node; a rectifier electrically connected to thelow pass signal filter; an energy storage element electrically connectedto receive a rectified output of the low pass signal filter; a switchingdevice electrically connected to the energy storage element and to anoutput node; wherein the arc detection circuit is configured so thatwhen the input node is connected to an AC output voltage of theelectronic lamp ballast during normal operation of the lamp, the energystorage element is charged to a voltage less than a voltage required toclose the switching device, and wherein under a condition of electricalarcing from a lamp connector of the electronic ballast, the energystorage element is charged to a voltage at least as high as the voltagerequired to close the switching device.
 17. The arc detection circuit ofclaim 16, wherein the low pass signal filter comprises a filteringresistor and a filtering capacitor connected in series with one another.18. The arc detection circuit of claim 16, wherein the rectifiercomprises at least one diode.
 19. The arc detection circuit of claim 18,wherein the rectifier comprises a first rectifying diode connected inparallel with the filtering capacitor.
 20. The arc detection circuit ofclaim 19, wherein the rectifier comprises a second rectifying diodearranged in series with the energy storage element, the secondrectifying diode and the energy storage element together being connectedin parallel with the filtering capacitor.
 21. The arc detection circuitof claim 20, wherein the first rectifying diode is a Zener diode. 22.The arc detection circuit of claim 16, wherein the energy storageelement comprises a storage capacitor.
 23. The arc detection circuit ofclaim 16, wherein the switching device comprises a transistor connectedbetween the output node and ground.
 24. The arc detection circuit ofclaim 23, wherein the energy storage element is connected to a controlterminal of the transistor.
 25. The arc detection circuit of claim 24,wherein the switching device is a FET and the energy storage element isa capacitor connected to a gate of the FET.
 26. The arc detectioncircuit of claim 16, wherein the low pass signal filter is a low passnotch signal filter comprising a low frequency block signal filter. 27.The arc detection circuit of claim 26, wherein the low frequency blocksignal filter comprises a low frequency block resistor and a lowfrequency block capacitor connected in parallel with one another.
 28. Amethod of detecting and stopping an electrical arc from a ballastconnector, wherein a lamp is energized via the ballast connector by aresonant inverter including an inverter control circuit controllingswitching transistors to produce an AC voltage to a series resonant tankin the inverter, the method comprising the steps of: filtering the ACvoltage with a low pass filter to remove AC components at least as highas frequency components associated with normal operation of theinverter; charging an energy storage element from an output of the lowpass filter so that the energy storage element is charged to voltagehigher than a threshold voltage if frequency components of the AC outputvoltage lower than the frequency components associated with normaloperation of the inverter are present; and disabling the inverter if theenergy storage element reaches the threshold voltage.
 29. The method ofclaim 28, wherein the disabling step comprises disabling the invertercontrol circuit.
 30. The method of claim 28, wherein the disabling stepcomprises disabling at least one of the switching transistors.
 31. Themethod of claim 28, wherein the inverter is connected to a power factorcorrection unit, and wherein the disabling step comprises disabling thepower factor correction unit.
 32. The method of claim 28, wherein the KCinput voltage to the low pass filter is taken from an electrical pointof the inverter between the switching transistors and the seriesresonant tank circuit.
 33. The method of claim 32, wherein the ACvoltage is taken from an electrical point in the inverter between a DCcapacitor electrically connected to the switching transistors and theseries resonant tank circuit.
 34. The method of claim 33, wherein the ACvoltage is taken from an electrical point in the inverter connected toboth the DC capacitor and an inductor in the series resonant tankcircuit.
 35. A method for detecting an arc from a ballast connector,wherein a lamp is energized via the ballast connector by a resonantinverter including an inverter control circuit controlling switchingtransistors to produce an AC voltage to a series resonant tank in theinverter, the method comprising the steps of: sensing the AC voltage;filtering out components of the sensed AC voltage other than lowfrequency components corresponding to a rectification frequency of thearc to produce a filtered signal; rectifying said filtered signal;storing energy of the rectified filtered signal over a plurality ofperiods; and generating an arc detection signal based on the storedenergy.
 36. The method of claim 35, wherein the AC voltage is taken froman electrical point in the inverter between the switching transistorsand the series resonant tank circuit in the inverter.
 37. The method ofclaim 36, wherein the AC voltage is taken from an electrical point inthe inverter between a DC capacitor electrically connected to theswitching transistors and the series resonant tank circuit.
 38. Themethod of claim 37, wherein the AC voltage is taken from an electricalpoint in the inverter connected to both the DC capacitor and an inductorin the series resonant tank circuit.